1. Field of the Invention
The present invention relates generally to improvements in digital spread spectrum systems and, more particularly, pertains to a new and improved device and method for frequency acquisition and compensation for digital cordless direct sequence spread spectrum telephones.
2. Description of Related Art
Cordless telephones currently exist which provide good speech quality using analog frequency modulation in the 46/49 MHz band. These analog cordless phones are inexpensive and have become a common replacement for the traditional corded phones. However, analog cordless phones have a limited useful range, requiring the handset to be operated in close proximity to the base station. Also, analog cordless phones are susceptible to interference and lack adequate security for protecting the privacy of conversations.
One solution to improve cordless telephone performance is to use digital modulation and coding techniques. Digital modulation and coding offer more robust voice communication over a radio channel than analog modulation. Digital modulation also provides a more noise-free conversation by suppressing co-channel and adjacent channel interference. Additionally, with digital modulation and coding, effective scrambling codes can be added to improve phone security.
Another solution is to use the 902-928 MHz Industrial, Medical and Scientific (ISM) band for radio transmission and reception. The Federal Communications Commission (FCC) allows transmitters in this band to have increased power levels, as compared to the 46/49 MHz band, which increases the useful operating range. At the highest transmit power levels, which are needed by a digital phone to overcome interference, the FCC requires that the phone system implement spread spectrum modulation.
In Direct Sequence Spread Spectrum (DSSS) systems, such as digital cordless telephones operating in the ISM band, it is desirable to use very inexpensive crystal oscillator references in both the handset and base station in order to reduce overall system cost. However, the cheaper the crystals, the greater the inaccuracy in the oscillator frequency.